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Faded zinc plating protection by washing process

 

I work in a fluid components factory. Mainly dealing with pipes that undergo zinc plating process (mainly external coating). After the zinc plating they are oiled and wait for some time before being washed. But even waiting some time (4 hours), after I wash the pipes loose some of their chromium coating, turning to a different color (from yellow to kind of white). What can I do to prevent this problem? Do I have to increase the waiting time before washing? Or my zinc process is wrong? Does that happens to other process? thanks Rodrigo Slud

Rodrigo Slud
fluid connectors - Curitiba / Paran· / Brazil

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Rodrigo, zinc plating is attacked by strong acids and strong alkaline solutions. It sounds like the cleaner you use to wash the fitting is attacking the zinc plating. Typically, silicates are used as inhibitors in strongly alkaline cleaners to prevent them from attacking zinc. If your current cleaner is strongly alkaline, it may not contain enough silicate to inhibit the attack on the zinc. If this is the case, you should switch to either a less alkaline multimetal safe cleaner or a more inhibited alkaline cleaner.

You did not give the details of the washing process and whether there are rinse stages or not. If the cleaner is not rinse off, then a near neutral pH cleaner would be best. If the fitting are rinsed, check the rinse water for cleaner by titrating it as you would the cleaner but use a larger sample size (typically 100 mls). The more cleaner present in the rinse, the more likely the parts will develop a white color after drying. Also, the quality of the rinse water is important. The amount of chlorides and sulphates present should be kept as low as possible.

Roy Nuss
Trevose, Pennsylvania, USA

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2nd of three simultaneous responses-- If your parts are zinc electroplated and have a yellow chromate you should wait 24 to 48 hours for the chromate to cure before you do anything with the part.

Don Penzenik
- Elkhart, Indiana USA
 

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3rd of three simultaneous responses--  

When you have a zinc plated part with chromate, and it turns white, that is usually corrosion/rusting. The chromate is being "used up" and the zinc converts into white zinc oxide ⇦this on eBay or Amazon [affil links] . Are your parts being exposed to temperatures higher than 175 deg. F.? Are your parts being exposed to high humidity? If the white stuff is zinc oxide (and there is a good chance it is), then your pipes will exhibit poor corrosion-resistance properties (ie, show red rust quicker).

There is also the possibility that something is wrong with the plating process. Is your company doing the plating, or is it being provided by a supplier? If it is a supplier, call them up and get them involved. If your company is doing the plating, then call up your chemical supplier and get them involved.

Tim Neveau
Rochester Hills, Michigan